The invention relates to a sensor for analyzing gases.
Such sensors are known in many forms. For example, reference is made to the German Published Patent Application No. 42 43 734 A1, and in particular to the German Published Patent Application No. 43 11 849 A1, which proposes a sensor in which an oxygen pump cell is provided with a solid electrolyte and a measuring element, the pump cell being connected to the measuring element by way of a porous diffusion layer, via which the oxygen pumped by the pump cell is conducted to the measuring element. In addition, the oxygen to be pumped is supplied to the solid electrolyte via at least one diffusion channel leading laterally to the porous solid electrolyte.
Compared to the related art, the gas sensor of the present invention has the advantage that signal stability is improved and signal generation is accelerated. Because the outer electrode in the hot region of the gas sensor is in contact at least region-wise with the gas-permeable tunnel layer, which is directly exposed region-wise to the gas mixture to be analyzed, the gas to be analyzed can be supplied quickly and directly in a very advantageous manner to the outer electrode via the gas-permeable tunnel layer. In particular, the outer electrode is exposed more directly, for example, to an exhaust gas to be analyzed, and the potential is adjusted more quickly and reliably.
In this context, one skilled in the art understands the hot region to be the region of the gas sensor which is exposed to the measuring gas or in which the measuring signal is formed, while to be understood by the cold region is the region of the electrode leads which is exposed to markedly lower temperatures and essentially does not contribute to the formation of the measuring signal.
It is also very advantageous if the gas-permeable tunnel layer covers the outer electrode at least to a great extent, and in particular, if the outer electrode covered by the tunnel layer is at the same time surrounded by the substrate and the electrolyte layer. Disposed advantageously on the electrolyte layer is the MOx electrode, so that on the one hand, this electrode is exposed directly to the gas to be analyzed, and on the other hand at least substantially covers the electrolyte layer.
To ensure that the gas to be analyzed has access that is as simple, quick and direct as possible to the outer electrode, which is preferably a Pt outer electrode, the tunnel layer also very advantageously has a gas-intake surface which is not covered by the electrolyte layer and/or the MOx electrode, and therefore has direct contact with the measuring gas.
Furthermore, because the gas-permeable tunnel layer and the electrolyte layer separate the outer electrode from the MOx electrode which, for example, is disposed above it, the electrolyte layer can now be virtually gas-tight or merely slightly porous. Without the gas-permeable tunnel layer of the present invention, till now it was absolutely requisite that the electrolyte layer be gas-permeable, so that the measuring gas to be analyzed could diffuse through the MOx electrode and the electrolyte layer to thus reach the outer electrode. However, at the same time, this involved the disadvantage that because of diffusion processes through the porous solid electrolyte layer, in the course of time the outer electrode could become contaminated by the MOx electrode. The gas-permeable tunnel layer of the present invention overcomes this disadvantage, and the structure of the solid electrolyte layer can now be optimized independently of the previous requirement for gas permeability.
Furthermore, for special applications it is advantageous if the gas-permeable tunnel layer is not, for example, designed as a self-supporting cavity, but rather merely exhibits a markedly greater open porosity compared to the electrolyte layer. This permits higher mechanical stability, better process compatibility and avoidance of deformation during sintering.
In particular, the tunnel layer can also be advantageously made of the same material as the electrolyte layer, and differ from it merely due to an increased open porosity.
In addition, if necessary, the tunnel layer can advantageously also be doped with a catalytically active material that, in particular, contains a noble metal, a noble-metal alloy or a transitional metal oxide such as a manganese oxide. This catalytically active material is introduced in particular into the pores of the tunnel layer, and preferably has the form of nano-scale colloids. By adding the catalytically active material, a pre-catalysis of an exhaust gas to be analyzed is advantageously attained, and thus an even faster and better equilibrium adjustment.
Moreover, the tunnel layer can also be advantageously formed as a diffusion barrier which is permeable to specific gases, so that unwanted gas components from the measuring gas to be analyzed cannot reach the outer electrode.
Incidentally, the size of the tunnel layer is advantageously selected so that it is not covered by the electrolyte layer and the MOx electrode toward the cold side of the gas sensor.